The disclosure proceeds from an apparatus or a method for driving a solenoid valve. The subject matter of the present disclosure is also a hydraulic assembly having such an apparatus for driving a solenoid valve.
The prior art discloses apparatuses and methods for driving solenoid valves used in hydraulic brake systems having ABS and/or ESP functionality. Solenoid valves of this kind are embodied, in particular, as normally closed or normally open high-pressure switching valves and comprise a magnet assembly having a magnet coil, the magnetic force of which is generated by means of the electric current through the magnet assembly or the magnet coil. As a technical component, a solenoid valve serves to monitor the inlet or outlet of fluids or to control and to regulate the flow direction. The electric currents for driving the solenoid valves can be regulated or controlled here. The currents are typically set by means of an output stage that can be regulated in terms of current, wherein a PWM apparatus (PWM: pulse-width modulation) performs the regulation of the current through the magnet assembly by means of a duty ratio of the emitted PWM signal. The current setting is very precise, since the current is measured back and the duty ratio of the emitted PWM signal and hence of the current flowing through the magnet assembly are corrected as appropriate. When controlling the current, a PWM signal having a fixed duty ratio is merely set, which is intended to result in the desired current. In this case, disturbance influences cannot be directly affected. During valve movement, the valve armatures of solenoid valves, in particular of switching valves, generate an inductive coupling on account of their physical design in connection with the magnet coil, said inductive coupling influencing the current through the magnet assembly or magnet coil during this phase. The quicker the valve armature moves, the greater the induction. This results, during regulated driving of the valve, in the change in current, which is caused by armature movement during switching, bringing about automatic adjustment of the regulating parameters of the current regulation. This means that the current regulation aims to counteract the interruption in current and, at this point, the duty ratio of the PWM signal and the current through the magnet assembly increases. Without current regulation, the interruption in current by the armature movement has its greatest value, that is to say the current through the magnet assembly has its lowest value, at the highest armature speed shortly before the valve armature impinges on the pole core. The current regulation compensates fully for this effect, with the result that the valve armature experiences an additional force precisely at the moment it impinges on the pole core and generates a much louder switching noise than without current regulation. However, the variance between the desired and the set current without current regulation can be relatively large, since present ambient conditions or disturbance influences, such as coil temperature, which has an influence on the electrical resistance of a coil winding, and voltage, for example, have a major influence on the result (current setting tolerance). The desired current that is above a response threshold of the magnet assembly for triggering the switching process of the solenoid valve can thus no longer be ensured in some circumstances.
DE 195 29 433 A1 discloses, for example, a method and a circuit arrangement for monitoring an output stage module to which a multiplicity of inductive loads are connected. In this case, the current flowing through the inductive loads is regulated by clocked driving of the output stages and the current caused by the inductively stored energy in the off phases of the output stages is partially or temporarily led through a common current measuring device and is evaluated, including the inductive loads, for the purpose of checking the output stages. The inductive loads are, for example, electrically actuable hydraulic valves, wherein the compliance with predefined current values is checked in separate measurement processes. The compliance of minimum values of the valve attraction current and/or the valve holding current can thus preferably be monitored. Furthermore, the output stages, including the hydraulic valves, can be tested by driving the respective output stage for a predefined time period below the valve response time or in which a valve current below the valve response value is produced, and by evaluating the switch-off currents or freewheeling currents.